Protection of clear channel connections in communications networks

ABSTRACT

A method for protection of STM-n/OC-n clear channel connections calls for a protection diagram allowing protection of STM-n/OC-n clear channel signals transmitted from a source point to a destination point. This protection allows overcoming a fall along the path between source point and destination point. It can be used in different types of network structure, for example “ring” or “mesh”. The method calls for duplication of STM-n/OC-n signals at the transmitting end of the subnetwork and transmission over two different routes which can be defined as work and protection paths. To ensure success of the protection diagram the work and protection channels follow different routes. At the receiving end, the signal is selected from the work path or the protection path depending on the quality of the received signals, the protection state and the external commands.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to devices and subnetworksfor the protection of STM-N/OC-n clear channel connections.

STM-n/OC-n clear channel interconnection is the ability of a network ornetwork devices to interconnect an STM-n/OC-n signal as it is withoutprocessing in the case of STM-n signals, RSOH (Regeneration SectionOverhead) bytes and MSOH (Multiplex Section Overhead) as defined inITU-T specifications G.783 and G.707 (November 1995) or, in case of OC-nsignals, of the Section and Line Overheads as defined in BellcoreGR-253.

Assuming connection with an STM-n/OC-n signal of two network elements(NE A and NE C) passing through a third element NE B capable ofrealizing only VC-n/STS-n (not clear channel) connections, NE Bterminates the received STM-n/OC-n A and generates a new signal(STM-n/OC-n B) towards NE C with the same VC-n/STS-n within but withdifferent RS/Section OH and MS/line OH.

Even if no regeneration, grooming and consolidation operations arerequired on an STM-n signal traversing a conventional SDH/SONET node,i.e. a Digital Cross Connect—DXC—or an Add/Drop Multiplexer—ADM—,termination of the RS/Section Overhead and possibly termination of theMS/Line Overhead with resulting interconnection at the VC-n/STS-n levelare carried out nevertheless.

If the same connection is made over an NE B able to provide clearchannel connections (see FIG. 2), then the STM-n/OC-n A signal generatedby NE A will reach NE C without any modification.

In addition, some of the bytes of RS/Section OH and MS/line OH could beused for end-to-end transportation of data defined in standards (forexample the K1 and K2 bytes could transport the switching protocol forautomatic protection, the Di bytes could transport data communicationchannels) or proprietary data (for example “Media Dependent Bytes”,“Byte Reserved for National Use”, “Spare Byte”, etc.).

Processing of the RS/Section and MS/Line overhead bytes even if there isno need of regeneration, grooming or consolidation operations couldcause premature termination of these data. The STM-n/OC-n clear channelinterconnection allows this type of data to pass through the nodes whereno intrusive process is required.

But, at the present time, no protection system is provided forSTM-N/OC-n clear channel connections even though a sort of protection isrequired for this type of switched entity to protect the transmission ofthe STM-n/OC-n from the source point to the destination point or even toprotect any subnetwork connection of the STM-n/OC-n route.

SUMMARY OF THE INVENTION

The general purpose of the present invention is to remedy the abovementioned shortcomings by making available a method for protection ofSTM-n/OC-n clear channel connections.

Another purpose is to make available network elements and a subnetworkapplying the protection method.

It was therefore sought to provide in accordance with the presentinvention a method for protection of clear channel communications ofSTM-N/OC-n signals between two network elements (11, 12) of a subnetworkin which one element comprises a transmitting end and the other areceiving end in the subnetwork; the method including the steps ofduplicating the STM-n/OC-n signals at the transmitting end of thesubnetwork, transmitting the duplicated signals over two different pathsdefined as work (13) and protection (14) and selecting the signals fromthe work path or the protection path at the receiving end of thesubnetwork.

Again in accordance with the present invention it was sought to realizea clear channel subnetwork with protection of communications ofSTM-N/OC-n signals between two network elements (11, 12) in which oneelement constitutes a transmitting end and the other a receiving end inthe subnetwork characterized in that the element at the transmitting endcomprises a splitter for duplicating the STM-n/OC-n signals to betransmitted and means for routing the duplicated signals over twodifferent paths in the subnetwork defined work (13) and protection (14)and in which the element at the receiving end of the subnetworkcomprises a selector for selecting the signals from the work path or theprotection path.

BRIEF DESCRIPTION OF THE DRAWINGS

To clarify the explanation of the innovative principles of the presentinvention and its advantages compared with the prior art there isdescribed below with the aid of the annexed drawings possibleembodiments thereof by way of non-limiting example applying saidprinciples. In the drawings:

FIG. 1 shows a block diagram of a subnetwork applying the protectionmethod of the present invention in normal or work state, and

FIG. 2 shows the subnetwork of FIG. 1 in a protection state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the figures, FIG. 1 shows a clear channel subnetworkdesignated as a whole by reference number 10 which connects with anSTM-N/OC-n signal two network elements 11, 12 (NE A and NE B in FIG. 1).As the subnetwork is the clear-channel type, i.e. the connection is madethrough a number of network elements capable of realizing clear-channelconnections, the STM-n/OC-n signal generated by NE A will reach NE Bwithout change.

In accordance with the method, the protection diagram of the presentinvention calls for duplication of STM-n/OC-n signals at thetransmitting end of the subnetwork (the NE A element) and thetransmission on two different paths which can be defined work subnetwork13 and protection subnetwork 14. For duplication the transmittingnetwork element NE A will be equipped with an appropriate known signalduplicator 15 readily imaginable to those skilled in the art.

To ensure success of the protection scheme the work and protectionchannels will follow different routes.

The protection path can be used for permanent transmission of theSTM-n/OC-n signal (1+1 protection) or for transmission of extra trafficin case of a protection event (1:1 protection), as defined in ITU-TG.841, revision 10/98.

At the receiving end of the subnetwork protection (element NE B) thesignal will be selected from the work path or the protection path. Theselection will be determined by the quality of the signals received (forexample, if the signal is missing or deteriorated—SF or SD) the state ofprotection (i.e. the presence of a wait-to-restore time: see below)and/or external commands.

To make the selection, the network element NE B will be equipped with anappropriate selector 16 also known and readily imaginable to thoseskilled in the art.

Under normal conditions, the signal will always be selected from thework path 13 (FIG. 1). In case of failure of the work path (interruptionor unacceptable deterioration) the signal will be selected from theprotection path (FIG. 2).

The selector 16 can operate in a mode with or without restore. Theoperating mode with restore requires that, after protection switching,the selector selects the STM-n/OC-n from the work path when the latteris restored. When the operating mode with restore is selected, a fixedtime (wait-to-restore) must expire before the work channel is usedagain. The wait-to-restore time is programmable in the device.

The quality information is obtained by unintrusive monitoring performedby the selector on the STM-n/OC-n work and protection signals received.

As mentioned above, the selector can also respond to external commandswhich are used for example to prevent protection switching (protectionlock) or to force the selector to select the signal from one or other ofthe two paths (“forced or manual switch to worker” and “forced or manualswitch to protection”).

A hierarchy is advantageously imposed between the signal conditions(i.e. SF or SD), the external commands and the protection state (i.e.the presence and passing of the wait-to-restore time).

In this manner, if more than one switching criteria are presentsimultaneously, the selector is controlled by the condition, state orexternal command with the highest priority.

Hence, the purposes set out have been achieved by making available aneffective protection scheme for an STM-N/OC-n clear channel connection.

The protection scheme proposed for protection of the STM-n/OC-n clearchannel signals transmitted from a source point to a destination pointprovides for overcoming any failure along the path between the sourceand destination points. In addition, it can be used in different typesof network structure, for example ring or mesh.

As the signal is interconnected without any modification of the datatransported by the overhead bytes, the protection scheme described issingle-ended and no communication protocol (APS protocol) is necessaryto align the two protection ends, local and remote, with the protectionstate.

The above description of embodiments applying the innovative principlesof the present invention is given by way of non-limiting example of saidprinciples within the scope of the exclusive right claimed here.

1. A method of protecting clear channel communications of STM-n or OC-nsignals, the signals comprising overhead comprising, in the case ofSTM-n signals, Regeneration Section Overhead and Multiplex SectionOverhead bytes, and, in the case of OC-n signals, Section and Lineoverheads, between a transmitting network element and a receivingnetwork element in a subnetwork, the method comprising the steps of:duplicating, by the transmitting network element of the subnetwork, theSTM-n or OC-n signals as duplicated signals at the transmitting networkelement of the subnetwork; transmitting the duplicated signals over awork path and over a different protection path without changing theoverhead by transmitting the duplicated signals without processing ofthe overhead, each of the work path and the protection path comprisingat least one further network element; and selecting, by the receivingnetwork element of the subnetwork, the duplicated signals from one ofthe work path and the protection path at the receiving network elementof the subnetwork.
 2. The method in accordance with claim 1, in whichthe selecting step is performed on the basis of at least one of thefollowing criteria: quality of the duplicated signals received from thework and protection paths and state of the protection and externalcommands.
 3. The method in accordance with claim 2, in which data on thequality of the duplicated signals received is obtained by means ofunintrusive monitoring performed at the receiving network element. 4.The method in accordance with claim 2, in which the selecting step isperformed by setting up a hierarchy among at least two of the criteriaand, in case of simultaneous presence of a plurality of criteria, theselecting step is performed by controlling the criterion with thehighest priority.
 5. The method in accordance with claim 1, in which theprotection path is used for permanent transmission of the STM-n or OC-nsignal.
 6. The method in accordance with claim 1, in which theprotection path is used for transmission of extra traffic if protectionis not required.
 7. The method in accordance with claim 1, in which theselecting step is performed in a mode according to which, after theprotection path has been selected due to failure of the work path, STM-nor OC-n signal is selected from the work path when the latter isrestored.
 8. The method in accordance with claim 7, in which a fixedtime must expire before the work path is selected again.
 9. A clearchannel subnetwork for protecting communications of STM-n or OC-nsignals, the signals comprising overhead comprising, in the case ofSTM-n signals, Regeneration Section Overhead and Multiplex SectionOverhead bytes, and, in the case of OC-n signals, Section and Lineoverheads, between a transmitting network element and a receivingnetwork element in the subnetwork, the subnetwork comprising: thetransmitting network element comprising a splitter for duplicating theSTM-n or OC-n signals to be transmitted as duplicated signals, and meansfor routing the duplicated signals over a work path and a differentprotection path in the subnetwork without changing the overhead, therouting means including means for routing the duplicated signals withoutprocessing of the overhead, each of the work path and the protectionpath comprising at least one further network element; and the receivingnetwork element of the subnetwork comprising a selector for selectingthe duplicated signals from one of the work path and the protectionpath.
 10. The subnetwork in accordance with claim 9, in which theselector selects the duplicated signals on the basis of at least one ofquality of the duplicated signals received from the work and protectionpaths and a protection state and external commands.
 11. The subnetworkin accordance with claim 9, in which the selector comprises means for,after a protection switching, selecting the STM-n or OC-n signals fromthe work path when the latter is restored.
 12. The subnetwork inaccordance with claim 11, in which a fixed time must expire before thework path is used again.
 13. The subnetwork in accordance with claim 10,in which the selector comprises monitoring means for unintrusivemonitoring of the quality of the STM-n or OC-n signals received on thework and protection paths.
 14. The subnetwork in accordance with claim10, in which the selector comprises means for responding to the externalcommands to do one of the following: prevent protection switching andforce the selector to select the signal from a particular one of thework and protection paths.
 15. The subnetwork in accordance with claim10, in which the selector comprises hierarchy means for setting ahierarchy among criteria according to which the selector is controlledby the criterion with the highest priority in case of simultaneouspresence of a plurality of criteria.
 16. A communications networkcomprising a clear channel subnetwork for protecting communications ofSTM-n or OC-n signals, the signals comprising overhead comprising, inthe case of STM-n signals, Regeneration Section Overhead and MultiplexSection Overhead bytes, and, in the case of OC-n signals, Section andLine overheads, between a transmitting network element and a receivingnetwork element in the subnetwork, the subnetwork comprising: thetransmitting network element comprising a splitter for duplicating theSTM-n or OC-n signals to be transmitted as duplicated signals, and meansfor routing the duplicated signals over a work path and a differentprotection path in the subnetwork without changing the overhead, therouting means including means for routing the duplicated signals withoutprocessing of the overhead, each of the work path and the protectionpath comprising at least one further network element; and the receivingnetwork element of the subnetwork comprising a selector for selectingthe duplicated signals from one of the work path and the protectionpath.